It has long been recognized that delivering a drug to its therapeutic site of action within the central nervous system can be a very difficult task because of the numerous chemical and physical barriers which must be overcome in order for such delivery to be successful. A number of methods have been designed to overcome some of these barriers to central nervous system drug delivery as, for instance, the use of liposomes to surmount the blood-brain barrier. However, the disadvantages of a liposome delivery system, including low drug loadings, short duration of action, limited ways to manipulate the rate of drug release, poor storage stability, and problems with scale-up, have precluded the use of such a system. Another method to overcome some of the barriers to central nervous system drug delivery consists of chemically modifying the active drug to a form, called a prodrug, that is capable of crossing the blood-brain barrier, and once across this barrier the prodrug reverts to its active form. One example of such a prodrug delivery system consists of the neurotransmitter dopamine attached to a molecular mask derived from the fat-soluble vitamin niacin. The modified dopamine is taken up into the brain where it is then slowly stripped from its prodrug mask to yield free dopamine.
The most common method to surmount some of the physical barriers preventing drug delivery to the central nervous system has been through the use of pumps. A variety of pumps have been designed to deliver drugs from an externally worn reservoir through a small tube into the central nervous system. Although such pump delivery systems can be externally controlled to a certain degree, the potential for infection directly within the central nervous system is great and the exact site of action of the drug within the central nervous system is largely beyond control.
To be successful, it does not suffice just to deliver the drug within the central nervous system. The drug must be delivered to the intended site of action, at the required rate of administration, and in the proper therapeutic dose. Commercially, the Alzet osmotic mini-pump has become an acceptable, very useful, and successful means of delivering drugs at a controlled rate and dose over extended periods within the central nervous system. However, adapting this device to deliver the desired drug to discrete brain nuclei presents vast difficulties such as implanting cannulas directly within the designated brain regions.
Still another technique that has been developed to deliver neuro-active agents, such as neurotransmitters, to the central nervous system is with the use of neural transplants. Viable neuronal tissue can be implanted directly within discrete brain nuclei. The duration of substance delivery from the transplanted tissue does not present a problem because implanted tissue may survive for a long time in the host""s central nervous system. This technique surmounts a number of obstacles cited above, however, despite claims that neuronal grafts from fetal dopamine cells exhibit some of the autoregulatory feedback properties that are normally found in intact dopamine neuronal systems, the exact rate at which the neurotransmitters are delivered from neuronal transplants at their site of action can not be predetermined.
In 1817, James Parkinson described a disease which he termed xe2x80x9cshaking palsyxe2x80x9d. This condition is presently known as Parkinson""s disease and occurs in the middle-aged and elderly. While its onset is insidious, often beginning with tremor in one hand followed by increasing bradykinesia and rigidity, it is slowly progressive and may become incapacitating after several years. In idiopathic Parkinson""s disease, there is usually a loss of cells in the substantia nigra, locus ceruleus and other pigmented neurons, and a decrease of dopamine content in axon terminals of cells projecting from the substantia nigra to the caudate nucleus and putamen commonly referred to as the nigrostriatal pathway.
Some symptoms of Parkinson""s disease can be treated by the administration of L-3,4-dihydroxyphenylalanine (levodopa or L-dopa). L-dopa, the metabolic precursor of dopamine, is used for replacement therapy because dopamine itself does not cross the blood-brain barrier. However, it must be given in large doses of 3 to 15 grams per day because much of the drug is metabolized before it reaches the site of action in the brain. Alternatively, it is often given in combination with a dopa decarboxylase inhibitor, such as carbidopa, which prevents the metabolism of L-dopa until it crosses the blood-brain barrier. Its greatest effect is on bradykinesic symptoms. After about five years of treatment, side effects develop and the treatment becomes less and less effective even with increasing doses of the drug. These problems have raised the question of whether or not it would be possible to replace the lost dopamine by other means which would deliver the drug to its therapeutic site of action within the central nervous system.
The discovery that a unilateral lesion of the nigrostriatal pathway by the neurotoxin 6-hydroxy-dopamine produced an asymmetry of movement and posture in the rat, provided an animal model for Parkinson""s disease. This asymmetry of movement is employed in the rotometer model developed to measure rotational behavior induced by drugs that interfere with dopamine neurotransmission such as apomorphine. The characteristic apomorphine-induced rotational behavior is only observed in animals with a 90 to 95% reduction of dopamine levels in the striatum, and replacement dopamine in this tissue either by transplants of fetal dopamine-producing cells or adrenal medullary tissue results in significant decreases in apomorphine-induced rotational behavior.
Even though these approaches are well documented for experimental animal models, their use as therapy for neurodegenerative disorders such as Parkinson""s disease present a number of practical as well as ethical considerations. Not only is the use of human aborted fetal tissue a controversial issue, but this technique involves complicated surgical procedures. Furthermore, although clinical trials of adrenal and fetal tissue implants in Parkinsonian patients are being conducted, the mechanism and long-term efficacy of tissue transplants within the nervous system remain unclear and is still a matter of medical debate. The best theoretical approach for treatment of such central nervous system pathologies continues to be one which would deliver the biologically active agent directly to the damaged region of the central nervous system.
Although a number of different methods have been proposed and are presently being utilized for the delivery of pharmaceutically active compounds to the central nervous system (as used herein, xe2x80x9cnervous systemxe2x80x9d and xe2x80x9ccentral nervous systemxe2x80x9d are generally used interchangeably indicating that although one aspect of the present invention is to provide for a means of delivering a neuro-active agent directly into the central nervous system, another aspect is to provide for uptake of the microspheres according to the present invention by astrocytes wherever they may occur in the nervous system), there are sufficient disadvantages to each method that the need for delivering biologically active substances to the central nervous system still exists. The present invention addresses this need in a unique manner.
Broadly defined, the present invention relates, in part, to microspheres that have been developed as injectable, drug-delivery vehicles in which bioactive agents are contained within a polymer compatible with nerve tissues. As used with regard to the present invention, the term microsphere includes microcapsules, nanocapsules, microparticles, nanoparticles and nanospheres.
Microcapsules, microspheres, and microparticles are conventionally free-flowing powders consisting of spherical particles of 2 millimeters or less in diameter, usually 500 microns or less in diameter. Particles less than 1 micron are conventionally referred to as nanocapsules, nanoparticles or nanospheres. For the most part, the difference between a microcapsule and a nanocapsule, a microsphere and a nanosphere, or microparticle and nanoparticle is size; generally there is little, if any, difference between the internal structure of the two. In one aspect of the present invention, the selective uptake of microcapsules into astrocytes, the mean average diameter is less than about 45 xcexcm, preferably less than 20 xcexcm, and more preferably between about 0.1 xcexcm and about 10 xcexcm.
As used in the present invention, the microcapsule, or nanocapsule, has its encapsulated material (in the present invention this is a bioactive agent or drug) centrally located within a unique membrane. This membrane may be termed a wall-forming polymeric material. Because of their internal structure, permeable microcapsules designed for controlled-release applications release their agent at a constant rate (called a xe2x80x9czero orderxe2x80x9d rate of release). Thus, as used in the present invention, microcapsules include microparticles in general which comprise a central core surrounded by a polymeric membrane.
In addition, microspheres encompass xe2x80x9cmonolithicxe2x80x9d and similar particles in which the bioactive agent is dispersed throughout the particle; that is, the internal structure is a matrix of the bioactive agent and a polymer excipient. Usually such particles release their bioactive agents at a declining rate (a xe2x80x9cfirst orderxe2x80x9d rate of release), however such particles may be designed to release internal agents within the matrix at a near zero order rate. Thus, as used in the present invention, microspheres also include microparticles in general which have an internal structure comprising a matrix of bioactive agent and polymer excipient. Preferred polymers according to the present invention are biocompatible particles. A more preferred particle according to the present invention is one which is both biocompatible and biodegradable.
One preferred polymer employed in the present invention, poly(lactide-co-glycolide), has a number of advantages which render it unique to the method of the present invention. An advantage of this polymer is that it is similar to materials used in the manufacture of present-day resorbable synthetic sutures. Other advantages that this polymer shares with acceptable polymers according to the present invention is that this material is biocompatible with the tissues of the nervous system, including the central nervous system. Still another advantage is that this material is biodegradable within the tissues of the central nervous system without producing any toxic byproducts of degradation. A still further advantage of this material is the ability to modify the duration of drug release by manipulating the polymer""s kinetic characteristics, i.e. by modifying the ratio of lactide and glycolide in the polymer; this is particularly important because of the ability to deliver neuro-active molecules to specific regions of the brain at a controlled rate over a predetermined period of time is a more effective and desirable therapy over current procedures for administration. Microspheres made with this and similar acceptable polymers serve two functions: they protect drugs from degradation and they release drugs at a controlled rate over a predesired time. Although polymers have been previously reported for use in the microencapsulation of drugs, the physical, chemical and medical parameters of the microencapsulating polymer for neuro-active molecules to be used in nervous system implantation (such as implantation within the central nervous system) technique according to the present invention are narrow; there is no general equivalency among polymers which allows a polymer previously used for encapsulation of drugs to be freely exchanged for the polymers used to encapsulate neuro-active molecules for drug delivery to the central nervous system or for cell uptake according to the present invention. This is especially true when the site of utilization is the central nervous system.
Although the specifically named polymers described in the Examples contained within this description meet the criteria necessary for implantation within the central nervous system, other biocompatible, biodegradable polymers and copolymers having advantages having similar properties to poly(lactide-co-glycolide) may be substituted in its place. Examples of preferred polymers having the properties of biocompatibility and biodegradability include poly(lactide-co-glycolide) copolymer; polylactide homopolymer; polyglycolide homopolymer; polycaprolactone; polyhydroxybutyrate-polyhydroxyvalerate copolymer; poly(lactide-co-caprolactone); polyesteramides; polyorthoesters; poly xcex2-hydroxybutyric acid; and polyanhydrides. In addition to polymers having both biocompatibility and biodegradability that are used to synthesize microspheres for delivery of neuroactive agents into the central nervous system, non-biodegradable but biocompatible polymers may be used to synthesize microspheres for the second aspect of the present invention, that is for the uptake of microspheres by astrocytes. Such biocompatible but not biodegradable polymers include polydienes such as polybutadiene; polyalkenes such as polyethylene or polypropylene; polymethacrylics such as polymethyl methacrylate or polyhydroxyethyl methacrylate; polyvinyl ethers; polyvinyl alcohols; polyvinyl chlorides; polyvinyl esters such as polyvinyl acetate; polystyrene; polycarbonates; poly esters; cellulose ethers such as methyl cellulose, hydroxyethyl cellulose or hydroxypropyl methyl cellulose; cellulose esters such as cellulose acetate or cellulose acetate butyrate; polysaccharides; and starches.
Results obtained from a number of studies indicate that implantation of these neuro-active agent-containing microspheres provides a feasible method for prolonged release of the agent into the central nervous system. Moreover, the data obtained from studies involving dopamine as the encapsulated agent indicate that dopamine microsphere preparations have the potential of being employed as a source of transmitter replacement allowing diffusion of the microencapsulated dopamine directly into the central nervous system at a controlled rate for pre-determined periods of time assuring functional significance and at the same time remaining compatible with the central nervous system tissue. However, most surprisingly, the data indicate that microencapsulated dopamine injected into specific regions of the brain has the heretofore unreported ability to cause growth of nerve fibers. Thus, the method of placing the microencapsulated neuro-active agents, manufactured in accordance with one aspect of the present invention, has the potential of promoting the growth of those neural elements which are responsible for the production of endogenous dopamine within the central nervous system. Once growth has taken place and the neural fiber elements have matured and stabilized within their environment, they will continue to produce and release dopamine within the central nervous system thereby providing for the first time a potential cure for Parkinson""s disease.
Among the neuro-active molecules or agents which may be microencapsulated and administered according to the present invention are neurotransmitters; neuropeptides; and neurotrophic factors including such agents as norepinephrine; epinephrine; serotonin; dopamine; substance P; somatostatin; nerve growth factor; angiotensin II; corticotropin releasing factor; choline; acetyl choline; cholinergic neuronotrophic agents; basic fibroblast growth factor; acidic fibroblast growth factor; brain derived growth factor; nerve growth factor; insulin growth factor; transforming growth factorxcex2; epidermal growth factor; transforming growth factor; glial derived growth factor; estrogen; inorganics used for the treatment of depression such as lithium; gamma aminobutyric acid; gamma aminobutyric acid mimetics; oxytocin; phenylethyl amine; and interleukin-1.
Among the neurological conditions which may be treated microencapsulated neuro-active molecules being placed directly within the tissues of the central nervous system are spinal chord injuries, amyotorphic lateral sclerosis, Parkinson""s disease, Huntington""s Chorea, Alzheimer""s disease, epilepsy, and Tardive dyskinesia. Depending upon the disease to be treated, it may be advantageous to provide more than one microencapsulated neurotransmitter, neuropeptide and neurotrophic factor to the central nervous system. For example, as dopamine, cholecystokinin, and epidermal and basic fibroblast growth factors may all be involved in Parkinson""s disease, ultimately it may be advantageous when presented with a patient having the disease to provide a mixture of microencapsules containing two, or more neural-active molecules to the central nervous system (see Example 4).
In order to provide a more complete description and provide a greater understanding of the various aspects of the present invention, reference is made to the following examples.